Motor vehicle designers continually strive to create vehicles which have lower emissions of noxious and greenhouse gases than vehicles currently in use. One means of reducing vehicular emissions is to utilize alternative fuels. Commonly used fuels such as gasoline and diesel fuel, are mixtures of complex hydrocarbons which may also contain unwanted chemicals, such as sulfur. One form of alternative fuel available is LPG. LPG is primarily composed of propane, a three carbon hydrocarbon, and butane, a four carbon hydrocarbon. These hydrocarbons have a lower carbon to hydrogen ratio than gasoline or diesel fuel. Because the carbon to hydrogen ratio is lower, less carbon dioxide is produced in the burning of LPG than in the burning of gasoline or diesel fuel. The longer chain hydrocarbons of gasoline and diesel fuel are much more likely to produce unwanted particulate emissions in the exhaust gas. Relative to LPG, gasoline and diesel fuel do have two advantages, namely: (i) they are both liquids at STP (standard temperature and pressure), whereas under typical ambient operating conditions LPG must be stored in a pressure vessel to be in a liquefied state; and: (ii) gasoline and diesel fuel produce more energy per unit volume of fuel as compared to LPG, even when LPG is in a liquid state. This means that in dealing with LPG fueled vehicles, one must manage the difficulties encountered with temperatures and pressures far from the ambient range.
A key physical factor in the management of LPG fuel conditions is the liquid/gas equilibrium. The ambient conditions will dictate the mixture of LPG vapor and LPG liquid found in the fueling system. Additional measures must be taken to ensure the correct balance of liquid and vapor for the operation of the fuel consumer, as for example the internal combustion engine of a motor vehicle. For example, the ignition system of the engine can be designed to use either a gas phase LPG or a liquid phase LPG. Components may be added to the fueling system to either condense vapor into the liquid state or to ensure that all the liquid state has been evaporated and heated into a gaseous state, depending on which phase of the fuel is required.
FIG. 1 schematically depicts an exemplar prior art LPG fuel system 10 providing fuel to a consumer, for example the engine of a motor vehicle, the system being shown undergoing refilling of the fuel tank via a conventional filler neck.
A pressurized fuel tank (or vessel) 12 holds LPG fuel 14 in a liquid phase 14′ and a vapor phase 14″. The fuel tank 12 is equipped with a pressure relief valve 15, and may be equipped with a temperature sensor 16 and a pressure sensor 18. The LPG fuel 14 within the fuel tank 12 may be subject to external heat 20 as for example coming from the motor vehicle exhaust system, outside of the fuel tank 12, as well as heat 22 from components within the fuel tank 12, as for example, produced by a fuel pump 24. All of these sources of heat increase the temperature inside the fuel tank 12, thereby increasing the vapor pressure inside the fuel tank.
By way of example, contained within the fuel tank 12 are components that make up a fuel delivery system 26. These components may be simply a filter 28 at a lead end of the fuel line 30, or may be a fuel pumping system 32 connected to the fuel line 30, including, merely by way of example, the filter 28, the fuel pump 24 (typically engaged to boost fuel feed pressure when the pressure inside the fuel tank 12 is below a predetermined level), a check valve 34, a filter 36, and a fuel pressure regulator 38 so that a desired fuel pressure differential across the fuel pump is maintained. External to the fuel tank 12, the fuel line 30 connects with various safety and fuel conditioning components well known in the art (not shown) which are suitable to the particular fuel delivery application that pertains to the fuel consumer 40.
An LPG refilling source or bowser 42 is schematically shown connected by means of a bowser nozzle 44, to a pressure sealed release refilling fitting 46 of the fuel tank filler neck 48. The fuel flow 50 is from the bowser 42 through the refilling fitting 46 and into the interior of the fuel tank 12, wherein an internal fill level valve 52, as for example in the form of a float valve, provides automatic shut-off of the fuel flow when the liquid phase 14′ reaches a predetermined level in the fuel tank 12.
For rapid refilling to occur, the fuel pressure of the bowser nozzle 44 should be well in excess of the fuel vapor pressure within the fuel tank 12. As the fuel vapor pressure within the tank approaches the bowser nozzle fuel pressure, the rate of refilling decreases and, if the fuel vapor pressure becomes high enough relative to the bowser nozzle pressure, refilling may become impossible. Impossible to refill, or no-fill situations, in which fuel cannot flow from the bowser nozzle into the fuel tank because of excessive backpressure caused by the fuel vapor pressure within the tank, are highly undesirable. If such a no-fill situation is encountered, then a technique used in the prior art to overcome this problem is to cool the contents of the fuel tank down in order to reduce the vapor pressure inside the fuel tank. Methods of the prior art to do this include pouring cold water over the fuel tank or placing ice or wet rags on the fuel tank. Such methods can be difficult and time-consuming to implement, and may be unacceptable, impractical or unavailable, depending on the circumstances.
Concern over ability to refill the fuel tank is exacerbated for fuels having multiple chemical components of varying volatility. LPG and other fuels which are stored at vapor pressure typically have multiple chemical components, each having differing vapor pressures. Examples of high vapor pressure components which may be present in LPG fuels include: ethane, nitrogen and carbon dioxide; and manufacture or servicing may introduce air (or other contaminant gases such as nitrogen used for leak detection) into the tank, which may not have been completely purged out. The vapor pressure inside the fuel tank is the vapor pressure of the fuel mixture, however the individual chemical components may have a vapor pressure which is higher or lower than the vapor pressure of the mixture. If the vapor pressure of a chemical component is higher than the mixture, then the component will tend to remain in its gaseous phase and the concentration (mole fraction) of that chemical component will be higher in the vapor phase relative to the liquid phase. Conversely, if the vapor pressure of a chemical component is lower than the mixture, then the concentration (mole fraction) of that chemical component will be lower in the vapor phase relative to the liquid phase. The chemical composition of the vapor phase inside the fuel tank will typically be different in relation to the chemical composition of the liquid phase because the vapor phase will contain a higher concentration (mole fraction) of high vapor pressure chemical components relative to the liquid phase. As a result, the rate at which high vapor pressure chemical components can be withdrawn from the fuel tank is less when liquid fuel is extracted as compared to when fuel vapor is extracted. Accordingly, as a fuel tank is emptied, the final vapor pressure will be related to the ratio of the chemical components, and that will depend upon the ratio of the liquid fuel to fuel vapor extracted. If high volatility (high vapor pressure causing) chemical components have been favored to remain in their gaseous phase and therefore ‘compress’ rather than ‘condense’ as the pressure inside the fuel tank increases, ability to refill the fuel tank is adversely affected. If the fuel tank pressure approaches the bowser nozzle pressure before the fuel tank can be filled up, then it will not be possible to fully refill (refuel) the fuel tank. Thus, if high vapor pressure components are allowed to accumulate inside a fuel tank, then the rate of refilling will be slow, or refilling may even be prevented (a no-fill situation). This problem is exacerbated for the next refill if during the present refill, a relatively larger quantity of high vapor pressure chemical components are added to the fuel tank than will be removed during operation of the fuel consumer. Therefore, it is desirable to keep the concentration of high vapor pressure chemical components at low levels in the fuel supplied; however, this may impose increased fuel costs, and the desired low levels from the perspective of fuel tank refilling, may not always be met in practice.
In the case of fuels which are stored at, or near their vapor pressure, the pressure in both the bowser supply tank and the fuel tank being refilled (refueled) will be close to the vapor pressure of the fuel, and both tanks will contain a mixture of liquid fuel and fuel vapor.
Variables which can affect the likelihood of a no-fill situation include: 1) the pressure differential across the bowser; 2) the height of the liquid fuel level in the bowser supply tank, relative to that of the fuel tank being refilled (for example, the bowser supply tank may be located underground, whereas the fuel tank being refilled is typically located above ground); 3) the chemical composition of the fuel in the bowser supply tank (fuel vapor pressure varies with chemical composition and the feed pressure at the bowser nozzle, may be reduced if the bowser supply tank contains low vapor pressure fuel); 4) the temperature of the fuel in the bowser supply tank (a lower fuel temperature will reduce the vapor pressure in the bowser tank and hence the feed pressure at the bowser nozzle; 5) the chemical composition of fuel in the fuel tank being refilled (fuel vapor pressure varies with chemical composition and the backpressure at the bowser nozzle to fuel tank interface will increase if the fuel tank being refueled contains high vapor pressure fuel); and, 6) the temperature of fuel in the fuel tank being refilled (a high fuel temperature will increase the backpressure at the bowser nozzle to fuel tank interface).
Factors which can affect this sixth variable (the temperature of the fuel in the fuel tank being refilled) include: 1) ambient temperature (higher ambient temperature tends toward higher fuel temperature), 2) proximity of the exhaust system to the fuel tank (reduced separation typically results in increased heat transfer to the fuel tank), 3) engine load (a higher engine load can result in increased heat transfer from the exhaust system to the fuel tank, 4) airflow over the fuel tank (increased airflow results in better convective cooling), and 5) engine run time (a longer time may translate to more heat transfer to the fuel tank.
FIG. 2 is a graph 60 of probability 62 (as an increasing percent) versus pressure 64 (in bar), which exemplifies how refilling (or refueling) of an LPG fuel tank may be affected by the vapor pressure within the fuel tank. Distribution curve 66 represents a hypothetical probability distribution of bowser nozzle pressure of a bowser (or fuel supply station), and distribution curve 68 represents a hypothetical probability distribution of the fuel vapor pressure within an LPG fuel tank under prior art operational conditions, both immediately prior to commencement of refilling, and wherein point 70 represents a hypothetical maximum safe tank pressure. Both distribution curves 66, 68 are affected by factors such as ambient temperature and fuel chemical composition, which can vary from fill-to-fill and from market-to-market. By way of example only, to facilitate fuel flow from the bowser nozzle into the fuel tank, the bowser nozzle pressure should be greater than preferably about 5 bar or more over that of the fuel vapor pressure inside the fuel tank in order to facilitate rapid refilling of the fuel tank in a filling station environment.
Accordingly, what remains needed in the art of LPG fuel systems, is to somehow selectively modify the pressure differential between the bowser feed pressure of the fuel entering the fuel storage tank and the vapor pressure within the fuel tank so that rapid refilling is always assured.